Method of fractionating air



March 29, 1955 P. POOL 2,704,930

METHOD OF FRACTIONATING AIR Filed Sept. 29, 1953 EXPANSION VALVE 'i ,44 &

INVENTOR.

LEONARD P. POOL ATTORNEY.

United States Patent Ofi ice 2,704,930 Patented Mar. 29, 1955 2,704,930 METHOD OF FRACTIONATING AIR Leonard P. Pool, Allentown, Pa., assignor to Air Products Incorporated, a corporation of Michigan Application September 29, 1953, Serial No. 382,986 5 Claims. (31. 62-1755) The present invention relates to the fractionation of gaseous mixtures, particularly to the fractionation of air or similar mixtures of nitrogen and oxygen containing traces of hydrocarbon impurities. More particularly, the present invention relates to a method and apparatus for eliminating danger of explosion of accumulated hydrocarbon impurities in a zone of high oxygen concentration during the fractionation of air or similar mixtures of oxygen and nitrogen.

It is generally recognized that the air supply delivered in compressed condition at low temperature to an air fractionating column may and generally does contain minor proportions of objectionable impurities. These impurities include water vapor and carbon dioxide which do not present any problem insofar as the danger of explosion is concerned. However, the impurities also include carbon monoxide and various hydrocarbons such as methane, ethylene, acetylene and heavier molecular weight hydrocarbons which do form highly explosive mixtures with oxygen when in sufficient concentration. Such dangerous hydrocarbons may occur in minute quantity in the atmospheres of industrial districts and acetylene in particular is believed to be formed by the cracking of air compressor cylinder lubricating oils. Of the dangerous impurities, carbon monoxide has a boiling range below that of oxygen and the greater part of the carbon monoxide passes out of the fractionating column with the gaseous nitrogen. The remaining impurities are liquefied and solidified in the column and tend to accumulate in the liquid oxygen fraction in which they are suspended and perhaps to a slight extent dissolved.

In a two-stage air fractionating column, one stage being at a relatively high pressure and the second stage being at a relatively low pressure, crude oxygen and nitrogen fractions produced in the high pressure stage are passed through expansion valves and fed to the low pressure stage where product nitrogen and oxygen frac tions are produced. The usual two-stage fractionating column has its fractionating zones separated by a condenser-vaporizer with the vaporizer side exposed to the low pressure fractionating zone and the condenser side exposed to the high pressure fractionating zone.

It is the general practice of the art to shut down column operation when the hydrocarbon impurities which collect in the liquid oxygen product reach a concentration approaching the danger level. Thereafter, the vaporizer side of the condenser-vaporizer is drained to remove the dangerous impurities. Thus, for example, a concentration of greater than 0.5 p. p. m. of acetylene on the oxygen side of the condenser is considered a maximum concentration for safe operation and upon attaining such concentration as determined by periodic analysis, column operation is shut down and the liquid oxygen containing hydrocarbon impurities is withdrawn from the column.

Bleeding ofi hydrocarbon impurities'when concentrations near the explosive level are approached is not always an entirely satisfactory method of coping with the danger of explosion for several reasons. For example, although periodic analysis of the hydrocarbon content of the liquid oxygen on the oxygen side of the condenser should give a good indication of when the danger level is being approached, sudden contamination of the air by hydrocarbon impurities may bring the level of hydrocarbon impurities above its maximum suddenly. In such case, shutting down column operation and then bleeding otf these hydrocarbon impurities does not give a quick and effective means for taking care of this type of emergency situation. This is particularly true because of the time factor required for shutting down column operation and draining the oxygen side of the condenser. Further, upon bleeding the hydrocarbon impurities off and vaporizing the liquid oxygen containing them, the vaporization of the hydrocarbon impurities may be almost instantaneous and even if a maximum safe level of hydrocarbon impurity concentration existed before shutting down column operation, this sudden vaporization of the hydrocarbon impurities may lead'to a momentary high concentration resulting in explosion.

Accordingly, it is a principal object of the present invention to provide an improved method and apparatus for eliminating the danger of explosion due to the accumulation of hydrocarbon impurities in the product oxygen fraction during fractionation of air or similar mixtures of nitrogen and oxygen.

A further important object of the present invention is to provide an improved method and apparatus for eliminating the danger of explosion due to the accumulation of hydrocarbon impurities in the product oxygen fraction of an air fractionating operation which method and apparatus provide efiective means for immediately eliminating the danger of explosion upon approachingor passing a maximum safe concentration of hydrocarbons.

Still a further important object of the present invention is to provide a method and apparatus as aforesaid which method and apparatus not only eliminates the explosion danger within the confines of the column but also provides effective means for eliminating the danger of explosion upon withdrawal of the hydrocarbon impurities from the column and subsequent vaporization of the oxygen containing these impurities.

These and other objects will become more apparent upon considering the specific description of the present invention presented hereinafter together with the drawing wherein:

The single figure diagrammatically illustrates the principles of the present invention.

Referring to the drawing, 10 designates a conventional two-stage fractionating column. Column 10 consists of a high pressure section 12 and a low pressure section 14 separated by condenser-vaporizer 16. Each of the sections is provided with bubble plates and caps 18. Liquid crude oxygen collecting in pool 20 of high pressure section 12 passes through conduit 22, an expansion valve 24 and conduit 26 to an intermediate point in the low pressure section.

The high pressure liquid nitrogen collecting in pool 28 on shelf 30 normally flows by Way of conduit 32, expansion valve 34 and conduit 36, valve 38 being open and valve 40 closed, to the upper end of the low pressure section 14 as reflux liquid.

Gaseous low pressure nitrogen is withdrawn from the top of the column through conduit 42 as product nitrogen and its refrigeration value is recovered by heat exchange against the incoming air stream which enters the column via line 44 after passing through expansion valve 45. Since the heat exchange system for obtaining air feed at the proper pressure and temperature conditions forms no part of the present invention and since any conventional system may be used, no details are illustrated here.

Oxygen in a desired state of purity, ordinarily or over, collects on the upper side of condenser-vaporizer 16 after overflowing shelf 48, thereby forming pool 50. Product oxygen is withdrawn in liquid phase via line 52 through valve 54 and its refrigeration value is normally recovered by heat exchange against the air feed. This method of withdrawal of product oxygen is similar to that described in U. S. Patent 2,597,385 and possesses the attended advantages thereof but may be by any of the other conventional methods of withdrawal also described in that patent. When desired, product oxygen can be withdrawn in gaseous phase through conduit 46 controlled by valve 47.

Conduit 56 with valve 58 are provided as a means for draining the condenser of the liquid oxygen product collecting on the oxygen side of the condensers and around the tubes of the condenser.

In accordance with the present invention, a reservoir 60 is provided for collecting a supply of liquid nitrogen during column operation and prior to the time of accumulation of a dangerous concentration of hydrocarhens in the product oxygen. Valve 62 being closed by regulating valves 38 and 40, a small fraction of liquid nitrogen may be diverted into reservoir 60 without interfering with the purity of the oxygen product. Preferably, the liquid nitrogen is accumulated in the reservoir after column operation has been established so that the liquid nitrogen fraction will contain a maximum nitrogen content and the purity of the product oxygen will not be affected.

After accumulation of a reservoir of crude nitrogen and prior to, but near the time when the concentration of hydrocarbons approaches the danger level, oxygen production is stopped by closing valve 54 and column operation may be stopped by stopping the air feed. Thereupon valve 62 is opened and the liquid nitrogen in reservoir 60 is transferred via conduit 64 to the oxygen side of condenser-vaporizer 16 where it mixes with the liquid oxygen containing accumulated hydrocarbons in pool 66, Unless the liquid nitrogen fraction diverted has been maintained at a pressure higher than that in column section 14, reservoir 60 should be located above pool 66 so that the transfer of crude nitrogen may take place by gravity flow. Thereafter, by opening valve 58, line 56 is permitted to drain the entire contents of pool 66.

The function of the reservoir of liquid nitrogen is twofold. Since the crude nitrogen contains a high per cent of nitrogen which is an inert gas, allowing the same to flow into the column and mix with the product oxygen containing hydrocarbons in pool 66 dilutes and thus reduces the percentage of hydrocarbons present yielding a liquid of hydrocarbon content which is safe to withdraw and may be subsequently vaporized without the danger of explosion. In addition, because of the relatively low temperature of the crude nitrogen in the nitrogen reservoir, the condenser-vaporizer is cooled to a temperature which is low enough to permit safe drainage of the oxygen side of the condenser.

Since the reservoir of crude nitrogen is to perform two functions, namely, dilute the contents of pool 60 and cool the temperature of the condenser-vaporizer to a level more safe for drainage of the pool 60, the volume of crude nitrogen collected in the reservoir is of practical importance. Tied in with this question of volume and determinative of the answer thereof is the question of the hydrocarbon concentration at the point of stoppage of column operation. A safe operating limit for a column is generally considered to be a maximum of 0.5 p. p. m. acetylene concentration. Actual acetylene concentration is of course determined by periodic analysis. Assuming that at the time column operation was stopped, the acetylene concentration was 0.5 p. p. m. and allowing a suitable safety factor for dilution, a safe size for the nitrogen reservoir may be considered to be a tank having a volume equivalent to the volume of the oxygen side of the condenser-vaporizer 16.

I claim:

1. In an air fractionating operation in which compressed and cooled air is expanded and the efiluent of the expansion step is fractionated in two zones maintained respectively at a relatively high and a relatively low pressure, and in which crude oxygen and liquid nitrogen fractions produced in the high pressure zone are transferred to the low pressure zone where product gaseous nitrogen fraction and a pool of product liquid oxygen fraction are produced, the method of eliminating danger of explosion due to accumulated hydrocarbon impurities in the product oxygen fraction which comprises the steps of creating a reservoir of liquid nitrogen fraction, transferring liquid nitrogen from said reservoir to the pool of product liquid oxygen fraction when the concentration of accumulated hydrocarbons in the pool reaches a value considered unsafe for further operation, and then withdrawing the hydrocarbons of resulting diluted concentration from the fractionating operation.

2. In an air fractionating operation in which compressed and cooled air is expanded and the eflluent of the expansion step is fractionated in two zones maintained respectively at a relatively high and a relatively low pressure, and in which crude oxygen and liquid nitrogen fractions produced in the high pressure zone are transferred to the low pressure zone where product gaseous nitrogen fraction and a pool of product liquid oxygen fraction are produced, the method of eliminating danger of explosion of accumulated hydrocarbon impurities in the pool of product liquid oxygen fraction which comprises the steps of commencing the fractionating operation, establishing normal operating conditions including withdrawal of product oxygen fraction, thereafter diverting a minor fraction of the total liquid nitrogen fraction passing to the low pressure zone, creating a reservoir of the diverted liquid nitrogen fraction, interrupting the fractionating operation when the concentration of accumulated hydrocarbons in the pool of product liquid oxygen fraction reaches a value considered unsafe for further operation, transferring liquid nitrogen fraction from said reservoir to the pool of product liquid oxygen fraction, and then withdrawing the hydrocarbons of resulting diluted concentration from the fractionating operation.

3. in an air fractionating operation in which compressed and cooled air is expanded and the effluent of the expansion step is fractionated in two zones maintained respectively at a relatively high and a relatively low pressure, and in which crude oxygen and liquid nitrogen fractions produced in the high pressure zone are expanded and transferred to the low pressure zone where product gaseous nitrogen fraction and a pool of product liquid oxygen fraction are produced, the method of eliminating danger of explosion of accumulated hydrocarbon impurities in the pool of product liquid oxygen fraction which comprises the steps of creating a reservoir of expanded liquid nitrogen fraction, interrupting the fractionating operation when the concentration of accumulated hydrocarbons in the pool of product liquid oxygen fraction reaches a value considered unsafe for further operation, transferring expanded crude nitrogen from said reservoir to the pool of product liquid oxygen fraction, and then withdrawing the hydrocarbons of resulting diluted concentration from the fractionating operation.

4. In an air fractionating operation in which compressed and cooled air is expanded and the efiluent 0f the expansion step is fractionated in two zones maintained respectively at a relatively high and a relatively low pressure, and in which crude oxygen and liquid nitrogen fractions produced in the high pressure zone are transferred to the low pressure zone where product gaseous nitrogen fraction and a pool of product liquid oxy gen fraction are produced, the method of eliminating danger of explosion of accumulated hydrocarbon impurities in the pool of product liquid oxygen fraction which comprises the steps of creating a reservoir of liquid nitrogen fraction, interrupting the fractionating operation when the concentration of accumulated bydrocarbons in the pool of product liquid oxygen fraction reaches a value considered unsafe for further operation, transferring liquid nitrogen from said reservoir by gravity flow to the pool of product liquid oxygen fraction, and then withdrawing the hydrocarbons of resulting diluted concentration from the fractionating operation.

5. In an air fractionating operation in which compressed and cooled air is expanded and the efiluent of the expansion step is fractionated in two zones maintained respectively at a relatively high and a relatively low pressure, and in which crude oxygen and liquid nitrogen fractions produced in the high pressure zone are expanded and passed to the low pressure zone where product gaseous nitrogen fraction and a pool of product liquid oxygen fraction are produced, the method of eliminating danger of explosion of accumulated hydrocarbon impurities in the pool of product liquid oxygen fraction which comprises the steps of commencing the fractionating operation, establishing normal operating conditions including withdrawal of product oxygen fraction, thereafter diverting a minor fraction of the total liquid nitrogen fraction passing to the low pressure zone, creating a reservoir of the diverted liquid nitrogen fraction, interrupting the fractionating operation when the concentration of accumulated hydrocarbons in the pool of product liquid oxygen fraction reaches a value considered unsafe for further operation, transferring liquid nitrogen fraction from said reservoir by gravity flow to the pool of product liquid oxygen fraction, and then withdrawing the hydrocarbons of resulting diluted concentration from the fractionating operation.

References Cited in the tile of this patent UNITED STATES PATENTS 2,664,718 Rice Jan. 5, 1954 

1. IN AN AIR FRACTIONATING OPERATION IN WHICH COMPRESSED AND COOLED AIR IS EXPANDED AND THE EFFLUENT OF THE EXPANSION STEP IS FRACTIONATED IN TWO ZONES MAINTAINED RESPECTIVELY AT A RELATIVELY HIGH AND A RELATIVELY LOW PRESSURE AND IN WHICH CRUDE OXYGEN AND LIQUID NITROGEN FRACTIONS PRODUCED IN THE HIGH PRESSURE ZONE ARE TRANSFERRED TO THE LOW PRESSURE ZONE WHERE PRODUCT GASEOUS NITROGEN FRACTION AND A POOL OF PRODUCT LIQUID OXYGEN FRACTION ARE PRODUCED, THE METHOD OF ELIMINATING DANGER OF EXPLOSION DUE TO ACCUMULATED HYDROCARBON IMPURITIES IN THE PRODUCT OXYGEN FRACTION WHICH COMPRISES 